Insulation Planning Sheet

Work out the thickness, then check it against the space. The cavity is fixed — the material is the only thing you get to choose.

BFCBrilliance · bfcbrilliance.com/tools/insulation-calculator

The job

Location
Date
Where (wall / attic / floor)
Target R (from local code)
Material
R per inch
THICKNESS NEEDED
Space available
FITS?
Area
Volume

R per inch — common materials

Blown fibreglass: about 2.8
Fibreglass batt, standard: about 3.2 (range 2.9-3.8)
Open-cell spray foam: about 3.5
Blown cellulose: about 3.7 (range 3.2-3.8)
Fibreglass batt, high density: about 3.8
Polyiso rigid board: 5.6-6.5
Closed-cell spray foam: 6.0-7.0 — highest of the common materials

The cavity is the constraint

2x4 stud cavity: 3.5 inches
2x6 stud cavity: 5.5 inches
⚠ R-13 at 3.2/in needs: 4.06 in — MORE than a 2x4 cavity
So R-13 batts for 2x4 are: high density, about 3.7/in
3.5 in of standard batt: about R-11
3.5 in of high-density batt: about R-13
3.5 in of closed-cell foam: about R-23
5.5 in of standard batt: about R-18
R-49 attic, cellulose: about 13.2 inches
⚠ Compressing a batt: gives the R of that THICKNESS, not the label

Before buying

Target R looked up for YOUR climate zone and location in the building
Cavity depth measured, not assumed
Thickness checked against the depth — does it actually fit?
Material chosen to fit the space, rather than a batt chosen and squashed
AIR SEALING done first — it often matters more and costs less
Wiring and plumbing planned around, not stuffed over
Vapour control checked for your climate — this is where buildings get hurt
Thermal bridging through the studs considered
Ventilation and recessed lights checked in attics before covering anything
Blown material: bag coverage read at the DEPTH you need, not the bag's headline

The cavity is fixed. The material is the lever

R-13 needs 4.06 inches of standard batt and a 2x4 cavity is 3.5. That is a subtraction, not a guideline. In a fixed space the only way to more R is a material with more R per inch — which is exactly what high-density batts and foam are selling, and why they cost more.

Never compress a batt to fit

It does not give you the rated R. It gives roughly the R of that thickness of that material and often less, because compression collapses the air pockets doing the work. An R-19 stuffed into a 2x4 cavity is about an R-13 at R-19 prices. Buy the batt made for the cavity.

Air sealing first

Insulation slows heat moving THROUGH a surface. It does nothing about air moving through a gap. Sealing is usually cheaper than adding R-value and frequently makes more difference — and a carefully installed R-13 beats a badly installed R-19 regardless.